The present invention relates to the low and medium consistency refining of lignocellulosic material, and more particularly, to the control of the refining gap between relatively rotating refiner plates in such refiners.
Low consistency refiners for lignocellulosic material are used for developing fiber to increase surface area and fibrils and for cutting fibers to reduce their length. Low consistency refining was generally understood with respect to lignocellulosic material, as referring to a refiner fed by pumped slurry having a consistency of about 2-5% fiber. Modern pumping techniques accommodate consistency up to about 16% fiber (sometimes referred to as "medium consistency"). In these types of refiners, flow control is accomplished on the discharge of the machine, by a single throttling valve in a single discharge line. This is in contrast to the control of so-called high consistency refiners, where the feed is metered by a device upstream of the refiner. As used herein, "low consistency" should be understood as referring to pumped slurry with flow control at the discharge, as distinguished from high consistency with upstream metering.
Conventional two zone refiners maintain a common discharge from both refining zones and therefore, small differences in the refiner plate bar depth between the two zones or other factors can change the relative pumping capability of each zone. This can result in one zone pulling more than one-half of the total flow being supplied to the refiner which then provides uneven refining in the two zones since the thrust in the zones and the power applied is equal. Another deficiency is that the zone with the lower flow will have a smaller operating gap and therefore have a greater tendency for plate contact and increased wearing of the refining plate surfaces. This problem of uneven flow is particularly noticeable at material flows that are at the minimum volumetric capacity of the machine where operation may be desirable due to the lower refining intensities available at the lower flows.